This application claims priority from German Patent Application No. 101 49 018.6-24 filed on Oct. 4, 2001, which is incorporated by reference herein.
The invention relates to a method of regulating or controlling the feed force of a drilling device whose linkage is provided with a hydrostatic bore hole motor which may have an eccentricity.
Bore hole motors of this type are also known under the designation mud motor and comprise a housing with an external diameter which corresponds approximately to the linkage diameter. U.S. Pat. No. 6,173,796 describes such a bore hole motor. Its housing, which serves as a stator, has a thread on the inside and contains a rotor likewise having a thread whose number of turns is one turn less than the number of turns on the stator. The rotor is connected to the drive shaft of a tool and, for the purpose of directional boring, can have an eccentricity, for example one or more kinks.
Bore hole motors, for example water or mud motors, operate on the principle of displacing screw motors and are driven with the aid of a fluid supplied via the drilling linkage, for example a water-bentonite suspension (drive fluid).
If the bore hole motor (MUD motor) or the linkage is provided with an eccentricity, the linkage has to rotate during rectilinear boring in order to neutralize the eccentricity, for example a kinked motor housing. During curved boring, on the other hand, the linkage rotation is interrupted, the eccentricity is brought into the angular position (path angle) which is decisive for the predefined curved path, and the non-rotating linkage with the tool driven by the bore hole motor is forced into the earth or rock by the feed drive. Here, the problem arises that the rotating extraction tool exerts a torque on the linkage which has the effect of linkage torsion. This linkage torsion then leads to a more or less significant deviation from the angular position set on the linkage drive. In order to correct this deviation, first of all a measurement is required, in order to determine the actual position of the tool or of the eccentricity, and to set the angular position to a corrected value. This requires the bore hole motor to be stopped in order to avoid vibrations which distort the measured result, and a great deal of skill on the part of the operating personnel. In addition, there is no torsion when the borehole motor is stopped. The machine operator determines the deviation only after a specific boring length has been covered, and then has to correct the boring direction or the boring angle. This is time-consuming and leads to a xe2x80x9cmeanderingxe2x80x9d course of the bore, which leads to increased casing friction when a product pipe is pulled in.
Since the tool merely provides the extraction work, the linkage is connected to a feed drive which moves the linkage forward with a specific feed force. This feed force is normally set by hand in order to take account of different ground conditions. In the event of too low a feed force, for example in soft ground, the feed speed is too low and boring is uneconomic. In the event of too high a feed force, for example in rocky subsoil, it is by contrast possible for the bore hole motor to stop in the ground or in the rock. The drive fluid which continues to be supplied then emerges at high speed between rotor and stator into the surroundings of the drilling head andxe2x80x94in particular when a liquid/solid suspension is used as the drive fluidxe2x80x94leads to severe wear on the stator thread and on the rotor thread.
If the bore hole motor or the linkage is provided with an eccentricity for directional boring, according to the invention, directional accuracy can be improved by the eccentricity not being set to the desired direction but to an angle of attack xcex3 which compensates for the linkage torsion.
If xcex1 is the path angle which is required for the desired boring direction or curved path and to which the eccentricity is normally set with the linkage at rest, then the angle of attack is given by the following equation:
xcex3=xcex1xe2x88x92xcex2.
Here, xcex2 corresponds to the torsion compensation angle which necessarily results during boring. This is calculated in accordance with the following formula:       β    =                                        T            ·            1                                Ip            ⁢                          xe2x80x83                        ·            G                          ·                              180            xc2x0                    π                    +                        K          1                ·        1              ,
in which
T=torque of the bore hole motor using the motor characteristic curve as a function of the pressure of the drive fluid (bentonite suspension)
l=drilling string length
Ip=polar surface moment of 2nd order
G=shear modulus of linkage material
K1=correction factor for changing pipe cross-sections in the connecting area.
With the aid of this formula, it is possible, in spite of the continuously changing length of the drilling linkage (number of linkage sections), to compensate for the linkage torsion, so that the eccentricity that determines the actual path of the tool through the ground or a rocky subsoil exactly follows the planned run. Monitoring measurements and the continual readjustment, on the basis of these measurements, of the linkage, which does not rotate during curved boring, are not required in the method according to the invention; the result is fewer erroneous bores even in the case of unpracticed operating personnel, and a higher boring speed, since the expenditure on time for the monitoring measurements and the readjustment of the linkage in order to correct the boring direction as a result of the unavoidable torsion are dispensed with.
In order to avoid undesired stoppage of the bore hole motor, the invention proposes to regulate the feed force of the linkage as a function of the pressure of the drive fluid, for example a bentonite/water suspension, for the bore hole motor. This can be done by the liquid pressurexe2x80x94as close as possible to the pressure leading to a motor stoppagexe2x80x94remaining in a predefined tolerance range or else being kept substantially constant. The characteristic curve of the bore hole motor reveals the fluid pressure at which the motor stops. Taking account of the volume-flow-dependent pressure losses in the linkage, according to the invention it is possible to determine that pressure at a point outside the ground, for example in the area of the drive, at which there is a risk of a motor stoppage. The feed force of the linkage is regulated according to the invention in such a way that the fluid pressure at the bore hole motor does not reach this pressure, but also does not deviate too extensively from this, in order to be able to operate with the highest possible feed rate, that is to say optimally.
The feed force of the linkage is preferably regulated as a function of the pressure of the drive fluid for the bore hole motor in accordance with the formula
pM=ppxe2x88x92xcex94pGxc2x7nxe2x88x92xcex94pM,
in which
pM=pressure of the bentonite/water suspension at the bore hole motor
pp=pressure of the bentonite/water suspension at the high pressure pump
xcex94pG=pressure drop per linkage section
n=number of linkage sections
xcex94pM=pressure drop through machine, etc.
in such a way that the torque of the bore hole motor remains slightly, for example 2 to 5%, below the blocking torque. In this case, the blocking torque is to be understood to be that torque effective at the bore hole motor or tool at which the bore hole motor stops.
The combination of the torsion compensation according to the invention with the feed control according to the invention is particularly advantageous. Even during directed boring, this combination ensures a course of the bore which is suitable for the run, with an optimally driven drilling tool.